It has been usual to produce bevel gears with tapering tooth spaces and particularly to carry out the hob roughing of pinions, with fair accuracy of the profile generated by the hob roughing and with corresponding simplification of the final smoothing, offsetting operation, by generating a tool in one direction, which may be called forward to form one of the tooth flanks and then generating the tool in the reverse direction, which may be called rearward to form the opposite tool flank. In a recent development, the attempt has been made to achieve this by shifting the cutter head radially to the axis of its cradle. The radial shift produced the counter profile required for the tapering tooth space. The travel of the cutters about the axis which also travels through an arc because of its displacement is called rolling; and because usually the tool (cutters) and workpiece are arranged for confrontation in vertical planes, the relative motions between the two are sometimes referred to as upward and downward.
This prior attempt was generally successful but required very considerable time. The reason is that the radial shifting of the cutter head relative to the cradle axis, between the forward and backward generating, caused the cutter, which was rotating about the other axis of the knife head, to generate a circular arc positioned at considerable variance from the theoretically required profile of the flank; therefore, it was only possible to operate with cutter points of little breadth and, accordingly, with small generating feed. For the same reason this prior method did not generate truly accurate tooth profiles of proper form according to the predetermined mesh kinematics. Nevertheless, the machine required very complicated construction, which additionally interfered with rapid radial shifting of the cutter head.
Therefore, and mainly in order to reduce the working time, a new method was developed, heretofore. Reference may be made to U.S. Pat. No. 2,725,792 to Wildhaber, assignor to the Gleason works, issued Dec. 6, 1955. Both flanks of a tooth space were generated with one and the same cutter head; cutters and workpiece rolled relative to one another, first in one and then in the other direction; and the desired tapering of the tooth spaces in one direction was achieved by using different proportions of transmission for the generating motion in the one and other direction. In order to generate both tooth flanks with approximately the desired contact angles, spaced blade cutter heads were used, increasing and decreasing the profile angles between the forward and backward generating. A special mechanism of course was necessary for this purpose.
In addition, some further substantial problems were encountered. The use of spread blade cutter heads did not totally compensate for the change of the working angle produced by the changing ratio of roll. Also, when the angles of obliquity were small or particularly zero, accurate meshing of the bevel gears was not achieved, although the machine was very complicated, because of the kinematics required for generating the additional modifications of motions.
The invention further reduces the time required for performance of the various motions in the machine and the cost thereof, while it adds to the accuracy of the generated tooth profile with tapering tooth space. It achieves this by eliminating the change of rolling conditions and corresponding correction of profile angles of spread blade cutters, particularly for the hob roughing of the teeth space of outwardly increasing width, by the use of a very short train of drives for the generating and by omitting provision for introduction of additional drive elements, performing additional motions.
More specifically, the object is achieved with the use of a spread blade cutter head revolving about a first position of its axis for forward generating and hobbing of one tooth flank; subsequent swiveling of the expanding cutter head about a swivel axis by a certain center offset angle to provide a new position of the cutter head axis; and subsequent circulation of the cutter head, at the same ratio of roll, to generate the other tooth flank of the tooth space, by revolving about the newly positioned axis.